The subject invention is an apparatus and method for magneto-caloric refrigerator (MCR) offering improved energy efficiency, and reduced emissions of pollutants and greenhouse gases.
According to the U.S. Department of Energy, refrigeration and air conditioning in buildings, industry, and transportation may account for approximately 1019 joules of yearly primary energy consumption in the U.S.A. Air conditioning is also a major contributor to electric utility peak loads, which incur high generation costs while generally using inefficient and polluting generation turbines. In addition, peak loads due to air conditioning may be a major factor in poor grid reliability. Most of the conventional air conditioning, heat pumps, and refrigerators may achieve cooling through a mechanical vapor compression cycle. The thermodynamic efficiency of the vapor compression cycle is today much less than the theoretical maximum, yet dramatic future improvements in efficiency are unlikely. In addition, the hydrofluorocarbon refrigerants used by vapor compression cycle today are deemed to be strong contributors to the green house effect. Hence, there is a strong need for innovative approaches to cooling with high efficiencies and net-zero direct green house gas emissions.
The magneto-caloric effect (MCE) describes the conversion of a magnetically induced entropy change in a material to the evolution or absorption of heat, with a corresponding rise or decrease in temperature. In particular, MCE material may heat up when it is immersed in magnetic field and it may cool down when removed from the magnetic field.
All magnetic materials, to a greater or lesser degree, may exhibit an MCE. However, some materials, by virtue of a unique electronic structure or physical nanostructure, may display a significantly enhanced MCE, which may potentially be harnessed for technological application. In contrast to the MCE found in paramagnetic materials, the large MCE exhibited by ferromagnetic materials near their magnetic phase transition temperature (also known as the Curie temperature or Currie point) may render them suitable as working materials for magnetic cooling at the target temperatures appropriate for commercial, industrial, and home refrigeration application and heat pump devices, namely 200 to 400 degrees Kelvin. For example, gadolinium (Gd) is a ferromagnetic material known to exhibit a significant MCE near its Curie point of about 293 degrees Kelvin. In recent years, a variety of other MCE materials potentially suitable for operation at near room temperature have been discovered. See, for example, “Chapter 4: Magnetocaloric Refrigeration at Ambient Temperature,” by Ekkes Bruck in “Handbook of Magnetic Materials,” edited by K. H. J. Buschow, published by Elsevier B.V., Amsterdam, Netherlands, in 2008.
One of the very promising novel MCE materials is the intermetallic compound series based on the composition Gd5(SixGe1-x)4, where 0.1≦x.l≦0.5, disclosed by K. A. Gschneider and V. K. Pecharsky in U.S. Pat. No. 5,743,095 issued on Apr. 28, 1998 and entitled “Active Magnetic Refrigerants based on Gd—Si—Ge Materials and Refrigeration Apparatus and Process,” which is hereby incorporated by reference in its entirety. See also and article by V. K. Pecharsky and K. A. Gschneider, “Tunable Magnetic Refrigerator Alloys with a Giant Magnetocaloric Effect for Magnetic Refrigeration from ˜20 to ˜290K,” published in Applied Physics Letters, volume 70, Jun. 16, 1997, starting on page 3299. MCE produced by this family of compounds, also referred to as GdSiGe, has been labeled as “giant” because of its relatively large magnitude (reported as 4 to 6 degrees C. per Tesla of magnetic flux density). In particular, the MCE of the GdSiGe alloys may be reversible. Another noteworthy characteristic of the GdSiGe family is that the Curie temperature, may be tuned with compositional variation. This feature allows the working temperature of the magnetic refrigerator to vary from 30 degrees Kelvin to 276 degrees Kelvin, and possibly higher, by adjusting the Si:Ge ratio. For the purpose of this disclosure, an MCE material is defined as a suitable material exhibiting a significant MCE.
A magneto-caloric refrigerator (MCR) is a refrigerator based on MCE. MCR offers a relatively simple and robust alternative to traditional vapor-compression cycle refrigeration systems. MCR devices may have reduced mechanical vibrations, compact size, and lightweight. In addition, the theoretical thermodynamic efficiency of MCR may be much higher than for a vapor compression cycle and it may approach the Carnot efficiency. An MCR may employ an MCE material (sometimes referred to as a magnetic refrigerant working material) that may act as both as a “coolant” producing refrigeration and a “regenerator” heating a suitable heat transfer fluid. When the MCE material is subjected to strong magnetic field, its magnetic entropy may be reduced, and the energy released in the process may heat the material. With the MCE material in magnetized condition, a first stream of heat transfer fluid directed into a thermal contact with the MCE material may be warmed in the process and the heat may be carried away by the flow. When substantial portion of the heat is removed from the MCE material, the fluid flow may be terminated. As the next step, the magnetic field may be reduced, which may cause an increase in magnetic entropy. As a result, the MCE material may cool. A second stream of heat transfer fluid may be directed into a thermal contact with the MCE material where may deposit some of its heat and it may be cooled in the process. When substantial portion of the heat is deposited into the MCE material, the fluid flow may be terminated. Repeating the above steps may result in a semi-continuous operation. One disadvantage of such an MCR is the need for multiple flow loops typically involving pumps, heat exchangers, and significant plumbing.
Despite the apparent conceptual simplicity, there are significant challenges to the development of a practical MCR suitable for commercial applications. This is in-part due to the relatively modest temperature changes (typically few degrees Kelvin per Tesla of magnetic flux density) of the MCE material undergoing MCE transition. In addition, at present time the magnetic field produced by permanent magnets is limited to about 1.5 Tesla maximum. As a result, an MCR using permanent magnets and a single step MCE process may produce only a few degrees Kelvin temperature differential. Many important practical applications such as commercial refrigeration and air conditioning may require substantially higher temperature differentials, typically 30 degrees Kelvin and higher.
One approach to achieving commercially desirable temperature differentials from MCR may use multiple MCR stages (also known as cascades). Heat flow between stages may be managed by heat switches. Each stage contains a suitable MCE material undergoing magnetocaloric transition at a slightly different temperature. While the temperature differential achieved by one stage may be only a few degrees Kelvin, the aggregate operation of multiple stages may produce very large temperature differentials. See, for example, “Thermodynamics of Magnetic Refrigeration” by A. Kitanovski, P. W. Egolf, in International Journal of Refrigeration, volume 29 pages 3-21 published in 2006 by Elsevier Ltd., the entire contents of which are hereby expressly incorporated by reference.
A variety of heat switching approaches have been proposed but none has won commercial acceptance. For example, Ghoshal, in U.S. Pat. No. 6,588,216 entitled “Apparatus and methods for performing switching in magnetic refrigeration systems,” issued on Jul. 8, 2003, and incorporated herein by reference in its entirety, discloses switching of thermal path between MCR stages by mechanical means using micro-electro-mechanical systems (MEMS), and/or electronic means using thermoelectric elements. Ghoshal's thermal path switching by MEMS is inherently limited by the poor thermal conductivity of bare mechanical contacts. Ghoshal's thermoelectric switches have very limited thermodynamic efficiency which substantially increases the heat load to the MCR and reduces the overall MCR efficiency.
In summary, there is a need for 1) reducing or eliminating moving parts and pumped fluid loops in MCR systems, 2) simpler and more reliable MCR operation, and 3) means for attaining commercially desirable temperature differentials from MCR. A specific need exists for reliable, low-thermal resistance means for switching of the heat flow to and from the MCE material in staged (cascaded) MCR.